Stanford University researchers have an idea that may be something to sleep on.

If not enough sleep is bad, they wondered, could extended sleep be good?

They had a hunch that getting more than the usual amount would improve athletic performance and mood - a theory they say has not been explored by exercise physiologists.

The researchers asked five members of Stanford's women's tennis team to be their guinea pigs. After following a normal sleeping pattern for a few weeks, the students, ages 18 to 21, were asked to sleep longer; the goal was 10 hours a night.

The study was conducted during their regular tennis season; athletic performance and mood were measured after every practice session.

Cheri Mah, a researcher at the Stanford Sleep Disorders Clinic and Research Laboratory and lead author of the study, acknowledged that the athletes didn't always hit the 10-hour mark. But they noticed a difference even if sleep was extended by a half-hour, she says: The athletes' sprinting drills were faster, their hitting was more accurate and deeper, and their mood improved.

The study was part of a research abstract presented in Seattle last month at the annual meeting of the Associated Professional Sleep Societies.

Frank Wyatt, president of the American Society of Exercise Physiologists, says the study is too small to be definitive. But natural hormones are released into the brain during sleep that aid in the physical recovery process, he says.

"Serotonin and growth hormone are both released into the body while you sleep," he says, "These enhance your mood and facilitate tissue repair, respectively. So if you get extra sleep, you're going to have a better mood and have enhanced recovery."